This invention relates to the treatment of subterranean formations surrounding oil wells, gas wells, water wells, injection wells and similar bore holes
The flow of fluids from a subterranean formation to a well bore depends, among other factors, upon the permeability and flow capacity of the formation. Often permeability or flow capacity is not sufficient to allow a desired flow rate of fluids, such as crude oil or natural gas, from the formation. In such a case the formation can be treated to increase its production. These treatments include stimulating, gravel packing and polymer flooding.
Hydraulic fracturing is one type of stimulation treatment used to increase the production of fluids. Accordingly many methods have been developed which are useful for hydraulically fracturing a subterranean formation penetrated by a well bore. Commonly, in the art of hydraulic fracturing, a fluid is introduced into the formation sought to be fractured by a conduit, such as tubing or casing, disposed in a well bore. The fluid is introduced at a rate and pressure sufficient to produce a fracture in the formation and to extend the produced fracture from the well bore into the formation. The fluid can include a propping agent, for example sand, which results in placement of the propping agent within the fracture thus produced. Following the fracturing treatment, the introduced fluid is recovered from the formation but the proppant remains in the produced fracture to prevent complete closure of the formation. Thus, a highly conductive channel extending from the well bore into the formation is created through which formation fluids can easily flow.
Conventional fracturing fluids typically contain natural or synthetic water soluble polymers, which are well known in the art. Water soluble polymers viscosify aqueous liquids (used hereafter to mean any liquid containing some water) at relatively low concentrations due to their high molecular weight. However, very high molecular weight synthetic polymers are difficult to manufacture and tend to degrade when exposed to the high shear conditions encountered in petroleum recovery operations.
Gravel packing is another type of treatment used to increase the production of fluid from a formation. Unconsolidated formations, particularly those containing loose sands and soft sandstone strata, present constant problems in well production due to migration of loose sands and degraded sandstone into the well bore as the formation deteriorates under the pressure and flow of fluids therethrough. This migration of particles may eventually clog the flow passages in the production system of the well, and can seriously erode the equipment. In some instances, the clogging of the production system may lead to a complete cessation of flow or "killing" of the well.
One method of controlling sand migration into a well bore consists of placing a pack of gravel on the exterior of a perforated or slotted liner or screen which is positioned across an unconsolidated formation to present a barrier to the migrating sand from the formation while still permitting fluid flow. The gravel is carried to the formation in the form of a slurry, the carrier fluid being removed and returned to the surface. The proper size of gravel must be employed to effectively halt sand migration through the pack, the apertures of the liner or screen being gauged so that the gravel will settle out on its exterior, with the slurry fluid carrying the gravel entering the liner or screen from its exterior.
While numerous methods are available for effecting gravel packs in substantially vertical well bores, such methods often are unsatisfactory in effecting gravel packing of highly deviated well bores. Conventional gravel packing fluids utilizing uncrosslinked hydroxyethylcellulose, hydroxypropylguar, xanthan gum and the like as the viscosifier allows or permit the gravel to fall or settle to the low side of the tubing in long highly deviated wells. Such settling can result in a premature "sand-out" caused by a bridging of the settled particles across the tubing.
An ideal fluid for gravel packing operations would be one that shows little or no settling of gravel so that a high concentration of gravel can be transported through the tubing at any angle. The fluid also should exhibit adequate fluid loss to insure compact packing of the gravel against the formation face. The fluid also should "break" to a reduced viscosity fluid similar to the viscosity of water over a predesigned time interval and deposit no residual solids so as to avoid or minimize any formation damage.
A third type of treatment to increase the production of fluids from a formation is a flooding operation. Enhanced oil recovery (EOR) by flooding has become widely practiced by the petroleum industry. In conventional enhanced oil recovery processes, an aqueous flooding liquid is injected into the subterranean formation through a pattern of injection wells which surround one or more producing wells. The flooding liquid acts as an oil-immiscible front which displaces oil from the formation and forces it to the production well. In order to maximize the displacement efficiency of the flooding liquid, it has been a practice to add various materials to the medium to increase its viscosity.
As taught in "Encyclopedia of Polymer Science and Technology", Interscience Publishers, Vol. I, 192 (1964) it is known that the viscosity of an aqueous medium is increased by the addition of a water-soluble polymer. Such water-soluble polymers include polyacrylamide, acrylamide/acrylic acid copolymer, sodium polyacrylate, carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, polysaccharide as well as naturally occurring polymers such as guar and chemically modified guar such as hydroxypropyl guar.
Unfortunately, however, the aforementioned conventional water-soluble polymers suffer from many serious deficiencies or limitations in actual use in enhanced oil recovery. For example, for reasons of efficiency and economical considerations, it is common to employ very high molecular weight versions of such polymers. However, during the injection stage of the EOR . process (i.e., the pumping of the liquid into the formation), the aqueous medium containing the high molecular weight water-soluble polymer is exposed to high shear. Such shear often causes mechanical degradation of the polymer and thus reduces the viscosity of the aqueous medium. While lower molecular weight polymers are less sensitive to shear degradation, they must be used in much higher concentrations in order to achieve the desired level of viscosity.
In attempts to overcome some of the aforementioned deficiencies of the conventional water-soluble polymers, it has been a common practice to crosslink the polymer in order to improve resistance to thermal and shear degradation. See for example, U.S. Pat No. 3,247,171. Such attempts have generally not been successful. More recently, as taught in U.S. Patent No. 3,984,333, an aqueous medium has been thickened by dissolving a water-soluble block copolymer having water-soluble blocks and water-insoluble blocks in the aqueous medium. While such water-soluble block copolymers apparently exhibit reasonably good resistance to shear degradation, such polymers are difficult and often impractical to prepare. More importantly, such polymers do not exhibit significant tolerance of electrolytes normally present in the aqueous media to be thickened.
New synthetic polymers, known as interpolymers, are provided by the present invention and overcome the deficiencies of polymers currently used in stimulating, gravel packing, and flooding of subterranean formations. The interpolymers achieve the effect of high molecular weight polymers in solution with improved shear degradation properties, and generally, interpolymers are synthetic polymers that incorporate small amounts of hydrophobic groups into a polymer chain which is composed of water soluble monomers. The hydrophobic groups tend to associate with one another in the aqueous liquid, or associate with the hydrophobic portion of a surfactant present in the aqueous liquid. When hydrophobe association occurs, the solution viscosity increases relative to the same polymer without hydrophobic side groups. An additional benefit of some hydrophobe associative polymers is that the viscosity of the polymer is relatively insensitive to salts in the aqueous liquid due to the non-ionic character of the hydrophobic groups.
The synthesis of polymers and copolymers incorporating hydrophobic side groups is described in the art. Copolymers of acrylamide with water-insoluble alkylacrylamide and salts of acrylic acid and processes of making the polymers are disclosed in U.S. Pat. Nos. 4,520,182, 4,694,046, 4,673,716, and 4,528,348. Copolymers of acrylamide and alkylpoly(etheroxy)acrylate are disclosed in U.S. Pat. No. 4,463,152. Copolymers of acrylamide and a hydrophobe monomer to viscosify aqueous liquids are. disclosed in U.S. Pat. Nos. 4,524,175, 4,541,935, and 4,432,881. Additional hydrophobe associative copolymers are disclosed in U.S. Pat. No. 4,728,696.
Each of these patents disclose the utility of hydrophobe associative polymers in petroleum recovery operations because the polymers retain viscosity in aqueous solution when subjected to either shear, heat, or high salt concentrations or combinations thereof. Although hydrophobe-containing polymers with and without surfactants are an improvement over polymers without hydrophobes, the polymer systems do not provide sufficient proppant transport and viscosity characteristics for many well bore treatments. These characteristics are best obtained by chemically crosslinking polymers in aqueous solution to create relatively shear-stable and temperature-stable viscoelastic gels. The term "gel" is used hereafter to mean any crosslinked polymer solution.
The present invention combines the advantages of hydrophobe-containing polymers with the desirable characteristics of chemically crosslinked polymers to provide highly stable gels useful in treating subterranean formations. The novel crosslinkable interpolymers of the present invention overcome many of the deficiencies of previous hydrophobe-containing polymers by incorporating a vicinal diol-containing monomer, which is crosslinkable with a variety of polyvalent metal ions. The interpolymers (alone and in combination with hydrophobe-containing surfactants) provide viscosity at low shear rates that is significantly higher than polymers without hydrophobic groups. Additionally, the crosslinked interpolymers provide temperature stable gels with enhanced proppant transport qualities .